Bright finished aluminum alloy system



UnitedStates Pattltlo "ice 3,164,494 BRIGHT FHIESHED ALUMINUM ALLOYSYSTEM John B. English, La Grange Park, Ill, assignor to Reynolds MetalsCompany, Richmond, Va, acorporation of Delaware No Drawing. Filed (let.19, 1960, Ser. No. 63,503 15 Claims. (Cl. 148-315) This inventionrelates to chemical and electrolytic brightening of aluminum andaluminum alloys, and particularly concerns an improved aluminum alloyand metallurgical technique for obtaining a brightly reflective and yetrelatively heavily anodized article.

The production of decorative, anodized aluminum articles has been knownin the art for many years. The lustrous surface has an attractiveappearance, and the anodized coating overlying the reflective metallicsurface has the advantage of furnishing a hard-surfaced protectivecoating to preserve the bright aluminum substrate. Such an anodizedcoating is not itself reflective, but is generally transparent so thatlight can be reflected through it.

It has been found, however, when using conventional alloys andpractices, that the reflectance of an anodized article progressivelydiminishes to a noticeable extent as the thickness of the anodizedcoating is increased. In an effort to preserve most of the brightreflective qualities of the anodized article, while still retaining someanodized coating for protective purposes, it has usually been acceptedin the trade that the anodized coating should be limited to a thicknessof about 0.2 mil (0.002), even though it has been recognized that athicker coating would be desirable for better protection.

It is recognized, further, that a substrate of pure aluminum is superiorto its alloys when brilliance of surface appearance is desired. Thisresults, probably, from two circumstances: the brightness of an anodizedarticle depends not only upon the transparency of the coating, but alsoupon the surface regularity of the metallic substrate. Impurities oralloying constituents create the possibility of both inclusions in theanodized coating and pitting or other surface irregularities on thesubstrate. Of course, ultra high-purity aluminum is extremely expensive,and it lacks various physical properties which may be required foragiven application. The use of magnesium as a suitable alloying elementis known to yield satisfactory strength characteristics, but thequantity of that element has necessarily been restricted due to itsdeleterious effect on surface finish. 7

Limited by these competing considerations, art improvements haveemphasized the techniques of the finishing process, such as anodizing.In contrast, the present invention is directed to a solution of thefundamental problem concerning optimum alloy constituency of the basemetal to be anodized.

The object of this invention, then,'was to construct an alloy(preferably within the purity'limits attainable with commercialreduction cells) which would present an anodized surface comparable tothat previously produced only with higher purity aluminum. As acorollary, it was desired that the alloy be responsive to a wide rangeof commercial finishing processes.

In accordance with the present invention, it is possible to followconventional mill and finishing practices, in producing anodizedaluminum articles, but the thickness of the anodized coating can beprogressively increased with substantially less progressive decreases ofreflectivity than has been achieved in the art heretofore without theuse of refined aluminum. Basically, the invention derives from thediscovery that purity alone is not the controlling factor; that certainimpurity elements are much more critical than others; and that theremoval of all impurities 3,164,494 Patented Jan. 5, 1965 is simply anexpensive method employed of necessity because the critical elementswere not recognized.

More particularly, I have found that control of the manganeseconstituent, according to the instant teaching, is the dominant factorin obtaining a vastly improved result. Additionally, certain novel stepsin the manufacturing technique have been found to significantlyinfluence the response of the alloy to bright finishing procedures.

The control of manganese and other constituents in accordance with theinvention has proved broadly successful toward improving the types ofalloys which are presently preferred in the trade for brightlyreflective anodized articles, viz. the 5X57 series (Aluminum Associationdesignation).

The published compositions of several 5X57 alloys are given below.

Constituents 5357 5457 5557 .08 max- .10 max 10 mart .12 max 20 max 15max 15-.45 10-.40 Ma nesium -1 2 40-.80 Others, each 03 man .03 maxOthers, total 10 man.-- .10 max Aluminum al.

THE METHOD USED TO EVALUATE REFLECTIVE CHARACTERISTICS The lightreflecting characteristics of metallic surfaces can be used as a toolfor measuring and controlling quality and uniformity, and also as amethod of differentiating surface finishes. Visual methods of evaluatingsuch surfaces, though generally quite sensitive, may be influenced bypsychological factors such as color, physical surroundings, shape andpersonal preference. In addition, the sensitivity of the human eye andits resolving power affect the visual appearance of surfaces. With thisin mind, an instrumental technique has been developed which makespossible the measurement of small differences in the reflectancecharacteristics of surfaces.

When an incident light beam strikes a surface, the intensity,distribution and color of the reflected light are a measure of thenature of the surface. Instruments for the photometric measurement ofopaque surfaces typically consist of a suitable light source, anintegrating light sphere, a photoelectric cell, signal multiplier, andrecording or indicating equipment. The incident light beam is allowed tofall on the sample, and the reflected light is automatically integratedinside a magnesium oxide coated sphere. The average light density, asmeasured by the photometer, indicates the quantity of the reflectedlight. A suitable light source consists of an incandescent gas-filledtungsten lamp combined with a double conversion filter to give aluminous flux (incident light) equal in energy distribution to thesensitivity of the eye. Such a light source is employed in the modifiedGardner Pivotable Sphere Hazemeter which was used to determine thecharacteristics reported herein. The conversion filter employed consistsof two bonded glass plates (blue and amber) designed to give atransmittance curve closely approximating the visual response of equalenergy light at various wave lengths. With this filter, surfaces havingdiflerent lusters can be measured and the result compared with visualimpressions.

The pivotable integrating sphere allows the angle of incident light tobe varied from normal (or perpendicular) to the sample surface to asecond value which is usually less than 10 from the normal. In thesecond position, virtually all reflected light is retained inside thesphere. When the angle of incidence is 0 (perpendicular to the surface),only the diffuse or scattered light is retained I 3 t inside the sphereand the specular reflection is emitted outside the sphere.

The intensity of the reflected light from any particular light source,as measured on an instrument as described, may be evaluated under the.following definitions:

(1) Total reflectance (TR%): The percent intensity of the incident lightreflected from a flat opaque surface at all angles of observation. I

(2) Diffuse reflectance (DR%)z The percent intensity of the incidentlight reflected from a flat opaque surface at all angles and planes,other than the portion of the light reflected at an angle equal to theincident beam and in the same plane to the normal.

(3) Specular reflectance (SR%): The percent intensity of the incidentlight reflected at an angle equal to the incident beam and in the sameplane.

Thus, the sum of SR-l-DR=T R or specular and diffuse components equalthe total reflectance. In measuring these components, the angle ofincidence should be less than 10 (as measured from the normal).

A perfectly diffuse surface would scatter or diffuse the incident lightat all positions (angles) of the sphere giving identical readings forboth TR and DR. In contrast, a specular surface would reflectsubstantially all of the incident light, if the incident light is normalto the surface, and give extremely low DR. If the angle of incidence hasa value other than all of the reflected light (TR) is retained in thesphere. The two readings TR and DR, therefore, distinguish all surfacesif the illuminant and angle of incidence (for TR readings) are keptconstant. The difference TRDR, referred to as the specular component ofthe reflected light, is then readily calculated.

The generally adopted standard of reflectance consists of a speciallyprepared magnesium oxide surface. The preparation of this standard isoutlined in ASTM Specification D986-50. This surface (a perfectdiffuser) is considered to give 100% reflectance (TR% and DR%). In orderto balance the measuring circuit of the instrument, a black standard isused to define 0% reflectance. This standard consists of a black dyedvelvet surface pro viding a nearly perfect light trap and has anabsolute reflectance of 0.4%. Under some circumstances, it has beenfound to be advantageous to use an etched aluminum surface for an 80%reflectance standard, together with a black polished glass for 0%standard. This latter system of secondary standards is the basis uponwhich the data herein was determined.

Certain additional definitions are essential to an understanding of thesignificance of the accompanying data. Specular reflectance is definedas the ability of a plain surface to reflect an image withoutdistortion.

Specular reflectance factor SRF%) is a measure of the ratio of specularreflectance to total reflectance. By reporting the'TR and DR readings,the average reader will only wih difiiculty comprehend the realspecularity of a surface whereas the specular reflectance factor is adirect measure of the mirror qualities of the surface. As an example, asample of highly specular aluminum showed these readings as compared topolished chrome plate:

TR D R SRF percent percent percent Aluminum 87. 5 12. 5 90v 8 Chrome 67.5 9. 5 90. 9

The following Values may then be calculated for the standard silvermirror and the previously mentioned aluminum and chrome surfaces:

1 Arbitrarily; V

From the above discussion, it follows that specular surfaces should bereported in terms of specular reflectance (SRFEimage quality) andspecular brightness (SBFEirnage brightness As an additional example, twocommercial mirrors (silver and lead sulfide) gave identical SRF readingsof 96% but had SBF values of 92% and 45% respectively.

The following equations are employed to compute the two factors usedherein to differentiate the various sur- T R- TR D R faces discussed:

-DR DR SRF%- 100- R g or if the instrumental correction factor C hasbeen determined using the specular standard,

where TR and DR are the previously defined reflectance readings; TR andDR are the corresponding readings for the standard; and

DR TRDR V The following examples furnish specific illustrations ofpresent preferred embodiments of the invention.

Example I An alloy was made consisting of .03% copper, .08% iron, .05%silicon, 03% manganese, and 0.80% magnesium, the balance beingsubstantially aluminum (other constituents each being no more than .02%and totaling less than 05%). A standard sheet ingot of this alloy wascast by conventional D.C. casting methods, and homogenized in thetemperature range 1100-4175 F. for a period of several hours in order toobtain satisfactory grain size control and uniformity of structure. Theingot was then scalped to remove surface irregularities and thenhot-rolled (750900 F.) in a slabbing mill. The slabbed ingot was thencooled to about 525575 F. and given an additional reduction in thicknessWhile the metal temperature was above about 350 F. The ingot thicknesswas reduced from approximately 15 inches to about 1 inch in thehot-rolling step, and to about 0.15 inch in the subsequent rolling.Thereafter, the sheet was brought to a finished gage of about .040 inchby means of a conventional cold-rolling operation. Portions of the sheetwere tempered to approximately H25 condition, While others wereannealed. The sheet portions were finished by conventional mechanicalbuffing, then solvent degreased, bright dipped in a nitric acidsolution, and anodized with 15 %18% sulfuric acid in deionized water at6575 B, using 1015 amperes per square foot for different periods of time(ranging from about 8 minutes to minutes) for different specimens inorder to produce anodized coatings of various thicknesses on 5 v therespective specimens. The coatings were all sealed in deionized water at2102l2 F.

The reflectance characteristics of the specimens are tabulated below.The values for O-temper material are given in Table 1, while Table 2presents similar information for H25 temper material.

TABLE 1 Image Reflecting Characteristics Approximate Thickness of AnodicCoating (Mils) Quality, Brightness,

SRF, Percent SBF, Percent For comparison, specimens of 5457 alloyproduced by conventional procedures typically exhibit much poorerchanacteristics as the coating thickness is increased to one mil, viz.down to about 79 (SRF%) and about 60 (SBF% However, by utilizing theprocedures as specified in Example I, especially the intermediatewarmcool rolling, the results for even 5457 alloy can be improved tomaximums of about 87 (SRF%) and 73 (SRF%) and minimums, respectively, ofabout 81 and 64. Thus, the remarkable characteristics tabulated in Tablel are attributable both to the improved fabrication practice and to theimproved alloy composition.

Certain comparisons again indicate the superiority of the alloy andpractice described in Example I. For example, tempered 5457 alloy(approximately half hard) exhibits" characteristics ranging down. toabout 80 (SRF%) and 64 (SBF%) as the coating thickness is increased toabout 1 mil. The corresponding values for 5357 alloy are about 66 (SRF%)and 50 (SRF%); and the characteristics for that alloy seldom exceed '73and 61, respectively, even for a 0.1 mil surface coating.

Another comparative reference of interest is high purity aluminum(generally referred to as four nine metal) which has an aluminum contentof at least 99.99% and is generally produced by expensive refiningprocesses. When this material is fabricated by conventional techniques,its image characteristics are typically about 90 (SRF%) and 78 (SRF%)for H25 temper. It is apparent, therefore, that the alloy andfabricating practice of the present invention are capable of producing asurface finish substantially equivalent to those previously attainableonly with more expensive high purity aluminum.

Example II An alloy similar to that in Example I was prepared in thesame manner, but having the following spectrograph i0 analysis:

Cu Fe Si Mn Mg Z11 Ni Qr Ti Al .07 .05 .01 .75 .00 .00 00 .01 BAL.

Table 3 presents the image characteristics which were observed in thecase of the Example II alloy.

TABLE 3 Image Reflecting Characteristics Approximate Thickness of AnodicCoating (Mils) Quality, Brightness, SRF, Percent SBF, Percent Thespecimens of Example 1.1 were produced in H25 temper, and therefore theresults shown in Table 3 correspond to those of Table 2. It is readilyapparent from a comparison of Tables 2 and 3 that the results achievedwith the alloys of Examples I and II are equivalent. It

should be noted that the increase in the copper content of the ExampleII alloy apparently had no deleterious effect on the brightness of thefinished sheet.

Analyses of variant forms of the Example II alloy, which showedcomparable characteristics, are the followmg:

thickness, while that of alloy (2) was .125 inch..

In order to evaluate the critical upper limit of copper, additionaltests were made using alloys within the general designation: aluminumand 0.601.0% magnesium, up to 0.12% iron, up to .08% silicon, manganese.03% max. and varying amounts of copper. It was found that .08% copperdid not detract appreciably from the bright ness of the finished alloy.In fact, as much as 0.15% copper was indicated to be satisfactoryprovided the metal was annealed; however, tempered stocks (such as H25)exhibited an undesired hazing or clouding when the copper contentreached this level.

In addition, the presence of small amounts of zinc, nickel, chromium,and titanium (within the broad limits of .03% each and 0.10% total) didnot significantly affect the brightness characteristics. Beyond suchlimits, how ever, additions of chromium have progressively detrimentaleifects upon surface brightness. For example, additions of .05 and 0.10%chromium to an alloy such as that of Example I result in characteristicsnot differing substantially from those attainable with conventionalalloy 5457.

Example III In order to evaluate the influence of iron and silicon,

as well as the general impurity level, an alloy was prepared in themanner described in Example I, having the following analysis:

Cu Fe Si Mn Cr Ti Al MglZhINi .12 .07 I .01 .75 .02 l .00 .01 .03 BAL.

It will be apparent that this alloy may be formulated even fromcommercial reduction cell aluminum, since the sneg re aluminum contentis only about 99.75% exclusive of the magnesium. The imagecharacteristics of H25 .material having the above analysis aresummarized in Table 4.

TABLE 4 Image Reflecting Characteristics Approximate Thickness of AnodicCoating (Mils) I Quality, Brightness,

SRF, Percent SBF, Percent Example IV In order to evaluate the influenceof magnesium upon the image characteristics of the finished metal, twoalloys were constructed, having compositions as follows:

Cu Fe Si Mn I Mg Zn Ni Cr Ti A1 (a) .06 .07 .05 .01 1.05 .01 .00 .01 .01BAL. (b) .02 .08 .05 .01 3. 22 .03 .00 .01 .02 EAL.

The image characteristics for H25 temper material having compositions(a) and (b) as noted above are shown respectively in Tables 5 and 6.

TABLE 6 Image Reflecting Characteristics Approximate Thickness of AnodicCoating (Mils) Quality, Brightness,

SRF, Percent SBF, Percent While these readings are somewhat lower thanthe corresponding values for other alloys according to the presentinvention, having a lower magnesium content, the results are no lessremarkable when comparison is made to results previously attainable. Forexample, an alloy having the following analysis:

I Cu Fe Si Mn .Mg Zn Ni Cr Ti Al .10 .00 .05 .01 3.06 .01 .00 .01 .00BAL.

produced in H25 temper by conventional fabricating procedures exhibitedsubstantially lower image characteristics,

'viz. from about 74 down to about 53 (SRF% and from about 60 down toabout 35 (SBF% as the coating thickness ranged from 0.1 mil to 1 mil.

It is apparent, therefore, that novel alloys such as those of Example TVare particularly suitable for end uses which demand both high strengthand good image characteristics. Previously, the image characteristics ofan alloy of comparable purity were sacrificed whenever improvement ofstrength was essential. Similarly, previous alloys constructed toexhibit optimum image characteristics were necessarily of lowerstrength. The opportunity to increase the magnesium content is,therefore, a significant advantage of the present invention.

By way of summation, the present invention encompasses an improvedfamily of alloys particularly receptive to chemical and electrolyticbrightening. Also within the scope of the invention is an improvedfabricating method typified by a warm-cool rolling step which has beenfound to contribute materially to the superior image characteristics ofthe novel alloys disclosed herein, as well as aluminorm and otheraluminum alloys generally. With respect to that aspect of the inventionwhich relates to fabricating procedures, it has been found that optimumdevelopment of image characteristics may be achieved by use of a novelstep between conventional hot and cold-rolling operations in thepreparation of the metal. The intermediate rolling step may be conductedwhile the metal is in the temperature range from about 300600 F.Preferably, the metal is cooled following hot-rolling to a temperaturein the range of about 525-575 F., and. a substantial reduction inthickness is accomplished before the metal temperature falls below about350 F.

The alloy concept of the present invention evolves from an appreciationof the influence upon image characteristics of the various impurityconstituents found in commercial reduction cell aluminum. For thepurpose of improving the image characteristics of bright finishedaluminum and aluminum alloys, it has been determined that the followinglimits are desirable: copper no more than about 0.15%, iron no more thanabout 0.12%, silicon no more than about .08%, others no more than about.03% each and 0.10% total. In a preferred form of the alloys accordingto the present invention, the limits are as follows: copper .01-06%,iron .01.10%, silicon .01- .08%, manganese 03% max, others .02% max.each and .05% max. total.

In order to achieve optimum physical properties of such alloys with aminimum sacrifice in image characteristics, it has been found to besatisfactory to include up to about 3% magnesium. In a preferredcomposition, the magnesium content ranges from about 0.401.20%, andoptimum results are obtained when the magnesium range is approximately0.60--l.0%.

While various present preferred embodiments of the invention have beendescribed, it will be recognized that the invention may be otherwisevariously embodied and practiced within the scope of the followingclaims;

I claim:

1. An aluminum alloy especially suitable for'chemical and electrolyticbrightening, consisting essentially of magnesium up to about 1.20%,copper, .01-.08%, manganese 03% max., iron .01-12%, and silicon.0l.08%,in weight percent, balance aluminum.

2. An aluminum alloy especially suitable for chemical and electrolyticbrightening, consisting essentially of magnesium up to about 3%,manganese .03% max., copper .01-06%, iron .01.l0%, and silicon .01-.08%,in weight percent, balance aluminum.

3. An alloy according to claim 2 wherein the magnesium content is about0.401.20%

4. An aluminum alloy especially suitable for chemical and electrolyticbrightening, consisting essentially of alu- 75 minum and 0.601.0%magnesium, manganese 03% max.,

copper .01.06%, iron .01.10%, silicon .0l.08%, in weight percent.

5. An article having a highly reflective and lustrous surface,comprising a transparent layer of aluminum oxide upon a substrate ofaluminum alloy, said alloy consisting essentially of magnesium up toabout 1.20%, copper .01- 08%, manganese .03% max, iron .0l.12%, andsilicon .0l.08%, in weight percent, balance aluminum.

6. An article having a highly reflective and lustrous surface,comprising a transparent layer of aluminum oxide upon a substrate ofaluminum alloy, said alloy consisting essentially of magnesiumGAO-1.20%, manganese .03% max., copper 01-06%, iron .01-.10%, andsilicon .01- .08%, in weight percent, balance aluminum.

7. An article having a highly reflective and lustrous surface,comprising a transparent layer of aluminum oxide upon a substrate ofaluminum alloy, said alloy consisting essentially of aluminum and0.60-1.0% magnesium, manganese .03% max, copper 01-06%, iron .01.l%,silicon .0l.08%, in weight percent.

8. An alloy of unrefined reduction cell aluminum and magnesium, saidalloy consisting essentially of magnesium up to about 3%; copper.01-0.15%; iron .0l-.12%; not more than about .03% manganese; silicon.0l.08%, in weight percent, balance aluminum.

9. The alloy of claim 8 having a transparent layer of aluminum oxidethereon.

10. The alloy of claim 9 further characterizedrby a specular reflectancefactor of about 90 to 93 and a specular brightness factor of about 76-79when said transparent layer of oxide is 0.1 mil thick and a specularreflectance factor of about 88-91 and a specular brightness factor ofabout 71-73 when said transparent layer of oxide is 1.0 mils thick.

11. An aluminum alloy consisting essentially of up to about 3%magnesium; .01-0.15% copper; .01-.12% iron; not more than about .03%manganese; and 01-08% silicon, in weight percent, the balanceessentially aluminum.

12. An aluminum alloy according to claim 11 consisting essentially of.03% copper; .08% iron; 05% silicon; .03% manganese; .80% magnesium, inweight percent, balance aluminum.

13. An aluminum alloy according to claim 11 consisting essentially of.06% copper; .07% iron; .05% silicon; .0l% manganese; .75 magnesium;.0l% titanium, in weight percent, balance aluminum. 1

14. An article having a highly reflective and lustrous surfacecomprising a transparent layer of aluminum oxide upon a substrate ofaluminum alloy, said alloy consisting essentially of up to about 3%magnesium; .0l-0.15% copper; .01-.l2% iron; not more than about .03%manganese; and .0l.08% silicon, in weight percent, the balanceessentially aluminum.

15. An aluminum alloy adapted to give a brightly reflective surface whenanodically coated and consisting essentially of magnesium up to about1.20%, copper .01- '.08%, manganese not in excess of 0.03%, iron .O1.12%and silicon .0l.08%, in weight percent, balance aluminum, said alloyhaving a specular reflectance factor in the range of about 84 to about93 and a specular brightness factor in the range of about 69 to about 79when anodical- 1y coated with a layer of aluminum oxide of from 0.1 to1.0 mil thickness.

References Cited in the file of this patent UNITED STATES PATENTS1,329,312 Roberts Jan. 27, 1920 2,063,022 Beck Dec. 8, 1936 2,067,076Craighead Ian. 5, 1937 2,076,575 Kempf et al Apr. 13, 1937 2,280,169Stroup Apr. 21, 1942 2,574,318 Burkhardt Nov. 6, 1951 FOREIGN PATENTS337,558 Great Britain Nov. 6, 1930 621,259 Great Britain Apr. 6, 1947877,873 France Sept. 14, 1942

1. AN ALUMINUM ALLOY ESPECIALLY SUITABLE FOR CHEMICAL AND ELECTROLYTICBRIGHTENING, CONSISTING ESSENTIALLY OF MAGNESIUM UP TO ABOUT 1.20%,COPPER, .01-.08%, MANGANESE .03% MAX., IRON .01-12%, AND SILICON.01-.08%, IN WEIGHT PERCENT, BALANCE ALUMINUM.